The work in this thesis was aimed at elucidating aspects of the coordination chemistry of molybdenum in the +5 and +6 oxidation states which are related to the function of molybdenum in biological systems. This work is divided into two sections. Part I (chapters 1 to 5) is concerned with the preparation, characterisation and infrared studies of molybdenum (V) and (VI) complexes. Part II (chapters 6 to 9) involves crystallographic studies made on selected compounds including full structure analyses of three molybdenum complexes. In chapter 2 the current state of the literature, as regarding molybdenum coordination chemistry and the involvement of molybdenum in biological systems, is summarised. Chapter 3 is concerned with studies made on model compounds. Molybdenum (V) and (VI) complexes were synthesised, characterised and studied using physical inorganic techniques. One section deals with a detailed study of infrared spectra of MoO₂X₂L₂ adducts (X = Cl, Br; L = monodentate aprotic ligands) including assignments of all vibrational modes down to 100 cm-¹. The preparation and characterisation of molybdenum complexes with ligands of biological interest is presented in chapter 4. Several new complexes of molybdenum (V) and (VI) with synthetic and naturally occurring amino acids were synthesised and characterised. Part I of the thesis is completed with chapter 5 which describes the experimental work carried out. In part II, chapter 6 deals with those methods and techniques which are used in the University of Canterbury Crystallographic Laboratory during the process of structure determination. Results are presented for preliminary studies made on compounds which were not subsequently further investigated. Chapter 7 presents the results of a structure analysis on the complex cis-dioxobis(butane-2,3-diolato)-molybdenum(VI)bis(butane-2,3-diol). In this structure it is shown that the chelate rings are nearly planar. Also the solvate molecules are attached to the central moiety and linked to one another by a system of hydrogen bonds which extends throughout the crystal. A new metal oxo system for molybdenum (V) is reported in chapter 8 which describes the structure analysis of the compound ammonium cis-dioxouramil-N,N-diacetatomolybdenum(V)monohydrate. This compound is found to contain a monomoic cis-dioxo group, the first such to be structurally characterised for molybdenum(V). Further it is found that the two glycinato rings of the ligand are attached to the molybdenum atom in the stereochemically more unfavourable trans arrangement. Chapter 9 describes the crystal structure of the compound tripotassium cis-trioxonitrilotriacetatomolybdenum(VI)monohydrate. This compound is shown to contain a cis-trioxo grouping and has the ligand in an arrangement in which the two glycinato groups are attached to the molybdenum atom in a cis arrangement. The molybdenum is not seven-coordinate as has been suggested; rather the third glycinato group is unattached to the molybdenum and contains an anionic carboxylate group.